This invention relates to anti-cancer medicaments that are specifically targeted to cancer cells by being linked to Erythropoietin (EPO) receptor ligands.
It is widely known in the field of oncology that the main disadvantage of chemotherapy is its inherent lack of specificity. Although anti-cancer drugs like Taxol™ are very successful in arresting cellular division in tumour cells, they also have the same effect on normal cells, giving rise to significant tissue and organ lesions, some of which are irreversible. Hence, it is essential to overcome this unspecific nature of chemotherapy so that the patient can tolerate it more and increase the efficiency of the drugs used.
EPO is a 30.4 kD Glycoprotein Hormone that is the main growth factor responsible for the regulation of red blood cell production in mammals (Erythropoiesis).
The primary site for the production of EPO in adult organisms is the kidney, although lower levels of EPO are synthesised by the liver and brain.
Synthesis of EPO itself in the body is regulated by oxygen tension in the body tissues, and is controlled by both positive and negative feedback signals. Low oxygen tension induces EPO production which in turn causes an increase in red blood cell production in the bone marrow. The enhanced oxygen supply to body tissues reduces EPO synthesis.
Recombinant human EPO is now readily available and is regularly used in the treatment of anaemia resulting from renal failure, harsh drug treatment like chemotherapy, and HIV-infection. The protein sequence for EPO, 193 amino acids in length, can be found in GenBank under Accession No. 1104303A (Jacobs et al. (1995) Nature 313,806-810), which is incorporated herein by reference in its entirety.
The protein sequence for the erythropoietin receptor, 508 amino acids in length, can be found in GenBank, under Accession No. AAA52403 (Jones et al. (1990) Blood 76,3135), which is also incorporated herein by reference in its entirety.
EPO circulates in the plasma at low concentrations (in pico-molar range) and binds to EPO-receptors (EPO-R) situated on the cell surface membrane. For many cells the receptor number is low, ranging from several hundred to several thousand of EPO-R per cell. Other cells can have high numbers of EPO-R in the order of 30,000 or greater.
The EPO Receptor is a 55 kD, (508 amino acid residue) transmembrane protein comprised of a 24 amino acid signal peptide, a 226 amino acid external segment, a 22 amino acid transmembrane segment, and a 236 amino acid cytoplasmic domain. This receptor is activated via a single EPO molecule bridging EPO receptor pairs. Binding of EPO to these EPO receptors causes a cascade of events including phosphorylation of the protein tyrosine kinase, Jak2. This in itself induces phosphorylation of the EPO receptors at 8 tyrosine residues. A vast array of other protein kinase signalling events are subsequently involved which ultimately cause the cell to express proteins involved in Erythropoiesis.
Recently Acs et al, Cancer Research 61, 3561-3565, 2001, reported that human breast cancer cells express the Erythropoietin (EPO) receptor. This result is confirmed herewith, but we surprisingly disclose additionally that other human tumours express the EPO receptor, which has led to the present invention.